In the above-mentioned applications, a variety of methods is provided for preparing polysilanes in improved yields and with high molecular weights and/or low polydispersities. These methods take advantage of both prior art knowledge and new discoveries based on recent mechanistic studies.
From the prior art, it was known that polysilanes could be prepared from Wurtz-type condensations in appropriate "inert" solvents such as aromatic and aliphatic hydrocarbons. The solid reacting surfaces enabling the polymerizations included the alkali metals, usually sodium or potassium and even alloys of these. From the mechanistic studies, it was also known that one mechanism for chain termination and, correspondingly, lower molecular weights than might be desired, was chain transfer to solvent, e.g., by hydrogen abstraction from activated C-H bonds. See, e.g., Zeigler, Polymer Preprints, 27 (1), 109 (1986), at 109, second column. While this was known to be a potentially important chain termination pathway, it was thought that other rate-affecting phenomena were greater contributors to loss of molecular weight, especially in normal addition systems where the monomer concentration when monomers were added to a dispersion of the solid were considered rate-limiting. The relative unimportance of chain transfer to solvent was demonstrated by the routine preparation of polymers with molecular weights &gt;10.sup.6 in simple alkane or aralkane solvents when the inverse addition method was employed. (Inverse addition refers to addition of solid reductant to monomer.) Other factors thought to be important included those influencing chain growth vs. chain initiation, polymer precipitation, and chain transfer to monomer in some cases.
Thus, for example, in Ser. No. 07/327,195, pages 24-26, published as WO87/06234 on Oct. 22, 1987, it is generically stated that solvents must not participate in chain transfer reactions. No differentiation is made among, for example, toluene, xylene, benzene, hexane, tetradecane, glyme, tetrahydrofuran (THF), etc., despite the different H-atom abstractabilities involved. All are mentioned equivalently as suitable "non-chain-transfer" solvents, i.e., as having H-atoms of sufficiently low abstractability to "not participate in chain-transfer reactions." In fact, toluene and alkanes are used predominantly in the work reported in this application despite the increased H-abstractability of these solvents vis-a-vis, for example, the equivalently mentioned benzene.
Moreover, in Example 9 of the latter application, the results of polysilane polymerizations using a sodium amalgam (1:1, Na/Hg) in toluene and toluene/heptane mixtures are shown. It is concluded on page 32 of Ser. No. 327,195 that the products from a polymerization using such an alloy are no different from those using the customary sodium per se.
Other workers have also utilized non-chain-transfer solvents as defined in this application or solid alloys without obtaining any advantageous effects on yield of high molecular weight polysilanes. Burkhard (U.S. Pat. No. 2,554,976), for example, utilizes benzene, a rigorously non-chain-transfer solvent, in combination with sodium. He reports "high" molecular weights, but the stated values are only 318-3200 for insoluble polymers (see, e.g., column 3, line 57, column 5, line 3, column 4, line 27, column 4, line 50, inter alia). Clark (U.S. Pat. Nos. 2,563,005 and 2,606,879) generically reports the possibility of using a eutectic alloy of sodium and potassium but reports no results. Clark employs toluene or xylene as solvents. The potassium/sodium eutectic has a composition 78/22 K/Na w/w. West (U.S. Pat. No. 4,260,780, Example I) describes the preparation of a poly(phenylmethylsilane co-dimethylsilane) using Na/K alloy (78% K) in THF. Molecular weights/yields are not stated. However, West et al. (U.S. Pat. No. 4,324,901) subsequently report that use of potassium in small amounts causes polymer degradation. Use of pure potassium or sodium-potassium alloy (78% K) is reported to result in extensive crosslinking to produce an insoluble and infusible polymer. West et al. conclude that the amount of potassium should not exceed 1% by weight (column 1, line 62-column 2, line 5). Peterson, Jr. et al. (U.S. Pat. No. 4,276,424) utilize lithium or lithium-alkali metal alloys to prepare polysilanes in THF. However, their method has a tendency to produce almost exclusively cyclic rather than linear polysilanes (col. 3, lines 10-12).
Heretofore, a preferred manner for preparation of polysilanes in terms of yields of high molecular weights and/or low polydispersities is that of U.S. application Ser. No. 07/327,195, e.g., as exemplified in its Table 2, reproduced below (alkane solvents):
__________________________________________________________________________ NORMAL vs. INVERSE ADDITION MODES IN POLYSILANE SYNTHESIS POLYMER ADDN YIELD MODAL MW --M.sub.n, --M.sub.w MONOMER SOLVENT MODE (%) (.times. 10.sup.-3) (.times. 10.sup.-3) __________________________________________________________________________ PhMeSiCl.sub.2 Toluene I 10 500 N 23 42, 15 Dodecane I 43 60 N 95 100, 6 4-anisyl MeSiCl.sub.2 Toluene I 4.7 30 N 18.5 2.7, 18.7 -n-dodecyl MeSiCl.sub.2 Toluene I 5.2 300 N 1.5 74, 470 __________________________________________________________________________
and in its Table 6, reproduced below (toluene solvent):
______________________________________ INVERSE vs. NORMAL ADDITION IN (PhMeSi).sub.n SYNTHESIS ADDITION INVERSE ADDITION NORMAL ADDITION RATE MODAL MW MODAL MW (Meq/Min) (.times. 10.sup.-3) (.times. 10.sup.-3) ______________________________________ 80 600 160 2000 3.4 320 600 640 2000,210 4.0 ______________________________________
As can be seen, heretofore, the inverse addition mode has been significantly preferable to the normal addition mode even using the state of the art methods of U.S. application Ser. No. 07/327,195.
Accordingly, there has remained a need to improve and/or facilitate prior art methods for preparing polysilanes, especially by the commercially preferable normal addition mode.